1. Field of the Invention
This invention relates to a traction type elevator system and more particularly to an improvement in wire rope oil aimed at elevating traction efficiency.
For better understanding of the wire rope used in the conventional traction type elevator system, reference is had to FIGS. 1 and 2 for explanation of the traction for such elevator system.
In the drawing, the numeral 1 designates a driving sheave for an elevator hoist, not shown and having a groove 1a. The numeral 2 designates a guide wheel and the numeral 3 a wire rope placed around the sheave 1 and the guide wheel 2. The numeral 4 designates a car suspended from one end of the wire rope or cable 3. The numeral 5 designates a counterweight suspended from the other end of the wire rope 3. The wire rope 3 is made up of strands 3a and a central flax rope 3b.
In this traction device, travel of the car 4 and the counterbalance 5 takes place through the wire rope 3 and based on friction between the groove 1a of the sheave 1 and the wire rope 3. The marginal sliding traction ratio of the wire rope 3 is given by the following equation (1). ##EQU1## where T1 is the wire rope tension at the side of the car 4, T2 the wire rope tension at the side of the counterbalance 5, .mu. a friction coefficient or traction coefficient between the groove 1a and wire rope 3, k a factor depending on the profile of the groove 1a and equal to ##EQU2## .alpha. the undercut angle of the groove 1a, .gamma. the angle of contact of the wire rope 3 with the groove 1a and .theta. the angle of contact of the wire rope 3 with the driving sheave 1.
It is observed that the traction ratio T1/T2 is changed with load and according as the car is accelerated or decelerated and becomes maximum when the car is raised under full load or when the car undergoes maximum acceleration during descent under no load.
For instance, considering a system in which the weight of the car 4 is equal to W1, the weight of the counterbalance 5 equal to W2, the maximum load is equal to W, with W2=W1+W/2, the maximum acceleration is equal to .alpha.g and in which the weight of the wire rope 3 etc. is neglected, the traction ratio is given by ##EQU3## when the car is accelerated while travelling upwards under full load, and by ##EQU4## when the car is accelerated while travelling downwards under no load. It is essential that the traction ratio of the equations (2), (3) be not is excess of e.sup.k.mu..theta. of the equation (1).
However, when the weight of the car 4 is reduced for the sake of material saving and reduction of manufacture costs, the traction ratio given by the above equations is increased. In addition, when the hoist is small-sized for the sake of material saving and reduction of manufacture costs, and the diameter of the driving sheave 1 is reduced the angle .theta. becomes small. For increasing the durability of the wire rope 3 and the drive sheave 1 and thereby improving safety and reliability of the traction system it is also necessary to reduce the angle .alpha. and if possible to reduce the angle .alpha. to zero, that is, to use a U-groove as groove 1a. Hence, for a larger traction ratio, the values of the coefficient k and the angle .theta. and hence the value of e.sup.K.mu..theta. in the equation (1) become small so that the traction as required may not be realized. By this reason, a demand has been raised on the rope grease having a higher traction coefficient .mu..
Polybutene used as base for grease is manufactured by heat cracking of naphtha from the petrochemical manufacture process to a C4 fraction from which butadiene is extracted to give a residual butane-butylene fraction which is then reacted in the presence of a catalyst to give a copolymer of isobutylene and n butylene, with isobutylene being the main component. It is a polymer with a medium molecular weight (so-called oligomer). Thus it is neither a low polymeric organic compound with a molecular weight less than about 300 nor a typical high polymeric compound with a molecular weight in excess of 10,000. More particularly, it is a liquid polymer with a molecular weight equal to 300 to 2,500. A compound obtained upon hydrogenating the terminal double bond of the compound also belongs to the polybutene group. Liquid polyisobutylene which is a low polymer of high purity isobutylene may have the molecular weight of the order of 8,000 to 15,000 which is larger than that of the polybutene fraction.
Since polybutene and liquid polyisobutylene are non-drying and superior in chemical stability tackiness and low volatility, they are used as additives for a variety of greases including wire rope grease or prepared oils and as basis for constituent carriers.
Recently, a demand has arisen for an oil having a high traction coefficient, especially an oil used with motion transmitting components. Most of the oils used for this purpose exhibit non tackiness and higher volatility so that they are not useful with components of the elevator system which are exposed because of volatility, although they are highly suitable for lubrication of closed-system components. It has been observed that the elevator wire rope is generally used for about ten years and is generally driven on the drive sheave about 10,000,000 times. The wire rope must operate under severe conditions where the oil supplied to the wire rope tends to be scattered and mixed with metal powders worn off from metal components and where the humidity is high. The wire rope grease is therefore required to be low in volatility, wear-resistant and excellent in tackiness, oxidation stability and protectiveness. In addition, there is an increasing demand for a wire rope grease having a higher traction coefficient.
A grease made from polybutene or liquid polyisobutylene as discussed above tends to have a higher traction coefficient, as described in the Japanese Patent Publication No. 47-35763 corresponding to the U.S. Pat. No. 3,608,385. However, this grease has been developed for improving the traction effect when used with a motion transmission system and is not usable satisfactorily as elevator wire rope.
Since polybutene or liquid polyisobutylene is essentially fluid, while being tacky in general, it tends to drip from the elevator wire rope placed on the drum when coated as it is on such wire rope. Therefore, it must be soft-solid or semi-solid or in the form of grease in order to prevent such dripping. To this end, however, the thickness must be of such kind and used in such amount that will not deteriorate the traction coefficient, and the oil must be of such type that will satisfy the aforementioned oiling process employed in the manufacture of the wire rope.